1. Field of the Invention
This invention relates to semiconductors and more particularly to integrated optoelectronic receiver devices and circuits incorporating photodetectors and MODFETs formed with SiGe layers.
2. Description of the Prior Art
The advent of fiber-optic communications technology has increased the demand for high-speed optoelectronic devices and circuits that operate with data rates of greater than 1 Gbit/sec. In particular, a growing market exists for local area networks and short-haul optical connections that operate at wavelengths of xcex=850 nm. It would be desirable to fabricate these circuits monolithically due to the lower cost of production and performance advantages over discrete components. It would also be desirable to fabricate such circuits entirely in a silicon-based technology due to the reduced cost arising from their compatibility with existing Si-based technologies including CMOS logic circuits.
In the prior art, GaAs has been the previous choice for monolithically integrated optical receivers operating at xcex=850 nm. This is due to the favorable intrinsic material properties of GaAs; the absorption length for 850 nm-radiation in GaAs is xcex1xe2x88x921=1 xcexcm, and the electron mobility in GaAs is roughly 8500 cm2/Vs at room temperature. J. S. Wang et al., IEEE Phot. Tech. Lett. 5, 316 (1993) demonstrated the fabrication of high-speed integrated photoreceiver circuits composed of GaAs metal-semiconductor-metal (MSM) photodiodes and MESFETs with xe2x88x923 dB bandwidths as high as 11 GHz at xcex=850 nm. Further improvement of GaAs-based receiver performance has been obtained using GaAs MSM photodetectors integrated with AlGaAs/GaAs modulation-doped field effect transistors (MODFETs). V. Hurm et al., Electron. Lett. 29, 9 (1993) demonstrated photoreceiver circuits of this type with xe2x88x923 dB bandwidths as high as 14 GHz at xcex=850 nm.
In order to replace GaAs, an integrated Si-based technology must have comparable performance to GaAs and a relatively low-cost process. However, the intrinsic material properties of Si are much less favorable compared to GaAs. The absorption length in Si for 850 nm-radiation is xcex1xe2x88x921=20 xcexcm, which is over an order of magnitude longer than in GaAs. Therefore, for a Si photodetector to have high responsivity it must have a thick absorbing region making the detector very slow, and for high speed the absorbing region should be very thin resulting in an extremely poor responsivity. For instance, Y. S. He et al., Electron. Lett. 29, 9 (1993) demonstrated the operation of a lateral p-i-n photodiode integrated with a Si NMOS technology with a responsivity of 0.48 A/W at xcex=870 nm, but with a xe2x88x923 dB bandwidth of only 900 MHz. Moreover, these results were only made possible by using ultra-high purity Si, and an extremely large bias voltage of 30 V. On the other hand, M. Y. Liu et al., Appl. Phys. Lett. 65, 887 (1994) demonstrated operation of a Si on insulator (SOI) MSM photodiode with an absorbing region thickness of only 0.1 xcexcm that had a bandwidth over 100 GHz, but a severely-degraded responsivity at xcex=780 nm of 0.0057 A/W. Improvements in the bandwidth/responsivity tradeoff of Si photodiodes are possible, for instance, in U.S. Pat. No. 5,589,704 which was issued on Dec. 31, 1996 to B. F. Levine, the responsivity of an MSM detector was shown to increase by a factor of close to 4 by roughening the surface of a Si epi-layer grown on an SOI wafer. However such a technique is limited to use with SOI substrates, and may not be suitable for practical applications due to the complexities of the roughening process. Si photodetectors are further hindered by the fact that the electron mobility in Si/SiO2 inversion layers is several times lower than GaAs at room temperature, and the frequency performance and gain of Si NMOS devices is considerably poorer compared to GaAs MESFETs.
In U.S. Pat. No. 5,525,828 which was issued on Jun. 11, 1996 to E. Bassous et al., it was noted that the speed and/or responsivity of Si MSM photodetectors could be increased by adding a certain percentage of Ge to the absorbing layer. Increasing the percent Ge-composition of Si1xe2x88x92xGex alloy decreases the absorption length, and increases the electron and hole mobilities thereby leading to potentially faster devices.
It has also been shown that field-effect transistors fabricated on Si/Si1xe2x88x92xGex layer structures offer considerable advantages over bulk Si transistors. For n-channel MODFETs incorporating tensile-strained Si/Si1xe2x88x92xGex quantum wells, frequency performance is considerably better than Si MOSFETs for a given gate length device. Similar performance advantages can be obtained in p-channel MODFETs fabricated on compressive-strained Si1xe2x88x92yGey/Si1xe2x88x92xGex quantum wells. For instance, M. Arafa et al., IEEE Electron. Dev. Lett. 17, 586 (1996) obtained unity current-gain cutoff frequencies of 70 GHz for 0.1 xcexcm gate length p-channel transistors fabricated on compressive-strained Si0.7Ge0.3/Si0.3Ge0.7 heterostructures. In U.S. Pat. No. 5,659,187 which was issued on Aug. 19, 1997 to F. K. Legoues and B. S. Meyerson, it was shown that a low-defect density layer of relaxed Si1xe2x88x92xGex, with arbitrary Ge composition can be grown on a lattice-mismatched substrate using an intermediate graded-composition buffer layer where strain has been relieved in the buffer layer or below via activation of modified Frank Read sources which is a mechanism to generate new dislocations. This work demonstrated the practicality of producing devices and circuits using Si/SiGe heterostructures grown on a SiGe graded composition buffer layer on Si substrates. Finally, U.S. Pat. No. 5,534,713 which was issued on Jul. 9, 1996 to K. Ismail and F. Stern showed that complementary logic circuits could be fabricated using high-mobility electron and hole channels fabricated in strained Si/SiGe layers grown on relaxed SiGe buffer layers.
Despite the apparent advantages of SiGe technology over bulk Si for photodetectors, MODFETs, and CMOS logic circuits, the concept of combining these devices to form integrated photoreceiver circuits has not been suggested, nor has a clear method for monolithically integrating these structures in such a way as to allow high-frequency operation and low cost Si manufacturing been suggested.
It is an object of the present invention to provide a simple means of monolithically integrating a photodetector with high speed and responsivity with a microwave transistor on a Si substrate in such a way as to allow high frequency performance better than Si and comparable to that achievable in GaAs.
It is a further object of this invention to provide a method for fabricating an optoelectronic integrated circuit using a process that is fully compatible with standard Si processing.
It is still a further object of this invention to provide a means for monolithically integrating a high-speed photoreceiver circuit with a practical CMOS logic manufacturing technology.
A design and method of fabricating an optoelectronic integrated circuit (OEIC) on a Si substrate is presented. This is achieved by using a high-quality, Si/SiGe heterostructure grown on a Si substrate to monolithically integrate a high-speed and responsivity SiGe photodetector and a modulation-doped field effect transistor on the same wafer. A typical layer structure consists of a Si substrate, a graded Ge-content Si1xe2x88x92xGex buffer layer, a thick, undoped relaxed Si1xe2x88x92yGey buffer layer, a Si quantum well, a Si1xe2x88x92yGey undoped offset layer, a doped Si1xe2x88x92yGey supply layer, and optionally a Si surface layer may be incorporated. A MODFET can be fabricated on the layer structure using mesa isolation, and by defining source, drain and gate electrodes, while an MSM photodetector can be created on the etched surface by depositing interdigitated Schottky electrodes on the surface of an exposed buffer layer. The electrodes are configured in such a way that the application of a voltage between adjacent electrodes creates an electric field that penetrates into the underlying layers. Light or radiant energy incident from the surface creates free carriers in the buffer layer that travel to the electrodes, creating a current signal that is proportional to the power of the incident light. By using a bias resistor in series with the photodetector, a voltage is created which can be coupled to the gate of the MODFET, which, when connected to an appropriate load, amplifies the original optical signal.
The key aspect of this invention is that the Si1xe2x88x92yGey buffer layer not only acts as a pseudosubstrate for the subsequent growth of a MODFET layer structure with carrier mobility greater than bulk Si, but also as a sensitive absorbing mediumxe2x80x94with much greater sensitivity to infrared radiation than bulk Sixe2x80x94for a high-speed photodetector, thus providing a dual advantage over bulk Si integrated device structures. Specifically, the absorption coefficient for bulk Si at xcex=850 nm is xcex1xe2x88x921=20 xcexcm, but for Si1xe2x88x92xGex with x=0.25, xcex1xe2x88x921=8 xcexcm, an improvement by a factor of 2.5. This improvement can be enhanced by increasing the Ge composition of the Si1xe2x88x92xGex alloy; for x=0.75, xcex1xe2x88x921=5 xcexcm, which is a factor of 4 times greater than in bulk Si. The increased absorption leads to more photogenerated carriers for a given material thickness, and can allow the absorption layer thickness to be decreased, therefore decreasing the transit time of the carriers, and increasing the speed of the detector. The photodetector speed is further improved by using SiGe alloys because both the electron and hole mobilities are increased compared to bulk Si. At the same time, strained-layer structures grown on the relaxed SiGe buffer layers have greatly improved mobility compared to bulk Si structures, mainly due to the band splitting caused by the strain. The electron mobility in modulation-doped, tensile-strained Si quantum wells grown epitaxially on relaxed Si1xe2x88x92yGey buffer layers can be a factor of 3 higher than standard Si/SiO2 inversion layers. Similarly, the hole mobility in compressive-strained Si1xe2x88x92z,Gez quantum wells grown on relaxed Si or Si1xe2x88x92yGey buffer layers, where z greater than y, similarly show considerable improvement over Si/SiO2 inversion layers. These improvements enable the fabrication of both n- and p-channel FETs that operate at higher frequencies, and have higher gain at a given frequency than Si MOSFET devices with the same gate length.
The present invention provides a design and method of fabricating a SiGe photodetector with improved speed and responsivity compared to bulk Si on the same substrate as a MODFET device that has improved gain and frequency performance compared to bulk Si, and performance comparable to that of GaAs, and thus providing a method of fabricating integrated photodetector circuits that are greatly improved over bulk Si, and comparable to those achievable in GaAs.
The invention further provides a means of optimizing the detector by varying the properties of the substrate material, by tailoring the Ge-composition in SiGe and the strain in the epitaxial layer structure, by varying the electrode materials, and by varying the photodetector and Si/SiGe MODFET designs. In this way, the invention is applicable to wavelengths other than xcex=850 nm, depending upon the photodetection mechanism, Ge-content and strain of the layers.
The invention further provides a means of monolithically integrating a high-speed photodetector and a high-mobility transistor (MOSFET or MODFET) using a Si-manufacturing process incorporating SiGe epitaxial layers.
The invention further provides a means of monolithically integrating a high-speed photodetector and MODFET technology with a manufacturable CMOS logic process, enabling an entire optical/analog/digital subsystem to be fabricated monolithically on a chip.